1. Field of the Invention
The field of the present invention is that of turbine engines and, more particularly, that of the lifetime of the components of these turbine engines which are subjected to high temperatures.
2. Description of the Related Art
The components of the hot parts of a turbine engine and, in particular, the turbine blades are subjected in use to extremely high temperature conditions and protections have been conceived to enable them to withstand these extreme conditions. Amongst the latter is included the deposition of a coating, called thermal barrier, on their external face, which protects the metal in which they are formed. A thermal barrier is generally composed of a ceramic layer of around a hundred microns, which is deposited perpendicularly to the surface of the metal layer. An underlayer made of aluminum, of a few tens of microns, placed between the ceramic and the metal substrate, completes the thermal barrier by providing the bond between these two components together with the protection of the metal of the blade against oxidation.
Ceramic has the drawback of only having a low thermal expansion, whereas the substrate that forms the blade is made of a metal of the superalloy type which has a high thermal expansion coefficient. The difference in thermal expansion is compensated by the formation of the ceramic in a columnar form, since the columns can separate from one another in order to adapt to the new width of the substrate.
One of the consequences of this is that oxygen, which is present in the gas flowing in the jetstream of the turbine engine, comes into contact with the aluminum of the underlayer and progressively oxidizes it. Aging of the thermal barrier is thus observed, which depends on the thickness of the layer of alumina produced. When it reaches a certain level of damage, the aluminum underlayer no longer fulfils its function of elasticity, delaminations appear and flaking of the thermal barrier occurs. The metal of the substrate is then no longer protected and the blade is in danger of being very rapidly degraded.
It is therefore important to know the level of damage to the thermal barrier and to forestall the appearance of this flaking. Many techniques have been conceived for monitoring the state of the thermal barrier and to know whether it is possible to maintain a blade in use. Amongst the latter, visual inspection, which only detects a defect when flaking has already occurred, infrared thermography or again piezospectroscopy are known. IR thermography has until now been used, as is the case for the patent application EP 1494020, only for detecting delaminations in the aluminum underlayer, which are precursors to the flaking, by the fact that they modify the local thermal properties of the product. However, it only works when a defect has already appeared. Piezospectroscopy measures the stress existing at the interface between the ceramic thermal barrier and the aluminum underlayer. As long as the thermal barrier adheres to the part, a stress is measured at this interface and it is known that the barrier is sound, whereas the absence of stress corresponds to the appearance of a fissure at this interface. Here again, the detection only takes place after the appearance of a defect.
The existing methods, which only work by detection of a defect, do not allow the remaining lifetime of a component to be forecast, nor an intervention on it before the damage becomes too severe.